1. Technical Field
The present invention relates to a semiconductor device, to a semiconductor memory device, and to a method of manufacturing a semiconductor device. More particularly, the invention is suitable for a dynamic threshold voltage (DT) metal oxide semiconductor (MOS) field effect transistor having a variable threshold voltage during operation.
2. Related Art
In JP-A-2000-114399 and JP-A-2002-353340, which disclose semiconductor memory devices according to the related art, methods have been disclosed in which an N-channel field effect transistor constituting a static random access memory (SRAM) is formed of a DTMOS field effect transistor. According to these methods using the DTMOS field effect transistors, since a channel region is connected to a gate, a threshold voltage at the time of turning on the transistor can become smaller than that at the time of turning off the transistor, so that operation can be performed at a low voltage, which results in a decrease in a consumed power during operation. Meanwhile, the threshold voltage at the time of turning off the transistor is equal to that of a typical N-channel field effect transistor and a leakage current generated at the time of turning off the transistor becomes equal to that of a typical SRAM. The consumed power does not increase in a standby mode.
In addition, in JP-A-2000-22160, a method of controlling a substrate potential by means of gate and drain potentials by inserting an auxiliary transistor sharing a gate electrode with an MOS transistor between a drain terminal and a substrate terminal of the MOS transistor formed on a silicon-on-insulator (SOI) substrate has been disclosed. According to the method of inserting the auxiliary transistor, when the auxiliary transistor is turned on, a current flowing from the gate to the source is removed while a body potential becomes equal to the drain potential, so that advantages of the DTMOS field effect transistor can be achieved while preventing the consumed power from increasing due to the leakage current flowing from the gate to the source.
However, in the DTMOS field effect transistor, since the channel region is connected to the gate, a leakage current flows from the gate to the source when the transistor is turned on, so that the consumed power increases. In addition, in a short-channel SOI transistor, since the body region is narrow, a resistance between a body portion and a body terminal increases so as to increase an RC time constant, so that it is difficult to achieve high-speed operation.
Meanwhile, according to the method of inserting the auxiliary transistor, which shares the gate electrode with the MOS transistor, between the drain terminal and the substrate terminal of the MOS transistor in order to remove the leakage current flowing from the gate to the source, the number of transistors is doubled in a semiconductor memory device such as an SRAM or the like, which causes an element area to increase. In addition, the above-mentioned method cannot provide any solution for solving the problems associated with the high-speed operation of the short-channel SOI transistor.